Priority of Dutch patent application No. 1,016,329, filed in the Netherlands on Oct. 4, 2000, is hereby claimed. That patent application is hereby incorporated by reference.
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The present invention relates to a method and apparatus for consolidating earth strata.
U.S. Pat. Nos. 4,471,540; 4,643,615; and 6,254,308; International Publication No. WO 00/50696; and Published Japanese patent application No. 10278292, published as publication No. 2000104242 on Apr. 11, 2000, are incorporated herein by reference.
When the bearing capacity of a soil is insufficient for carrying out a civil engineering project, such as the construction of a rail road or a motorway, it may be increased by forcedly driving out/withdrawing water from the weak earth strata (clay, peat). By withdrawing water the soil material will be pressed against each other and thus obtain an increased bearing capacity. As a consequence, the volume of soil body will decrease.
Forced consolidation techniques have been known for a long time. Nowadays drainage ribbons are often used, which are driven vertically into the ground with great force by means of a drive-in lance. On top of the ground level, incorporating the projecting end portions of the ribbons, a sand body is placed over which an airtight membrane is arranged. In the sand body a drainage pipe has been accommodated, to which a pump is connected, with which water and air is withdrawn from the sand body and thus from the drainage ribbons and the areas surrounding it. The weight of the sand body increases the pressure of the water to be withdrawn, in order to accelerate the process.
However, an objection is that during the forced consolidation process the terrain, due to the presence of the membrane, is not available for (preparations for) construction activities. A membrane moreover has to be bought, arranged and guarded and mostly be removed and discharged in a flow of waste material.
It is an object of the invention to improve on this. The present invention provides a method for consolidating an earth stratum situated in a subgrade by withdrawing water that is present. The method comprises the stepwise arrangement from the surface of the soil, at some depth, of vertical draining means, such as drainage strips, in the earth stratum, the arrangement of horizontal drainage means, such as drainage pipes, in the upper area of the earth stratum and connecting them to the vertical drainage means for water transfer between them, the arrangement of an air sealing layer directly over the horizontal drainage means and the closing of the surface of the soil, as well as the connection of the horizontal drainage means to a pump for discharge of the water and air. In this way the upper load relatively increases to the atmospheric pressure and the terrain is made available again for other activities. The pump may be very effectively active, as the sealing prevents drawing in of air from above the drainage means, at least reduces it to a large degree.
Preferably a trench is made from the ground surface and the vertical drainage means are arranged from the bottom of the trench. As a result the drainage means need not be pressed through an upper layer, and their length may be limited to the minimum necessary.
In an advantageous embodiment the trench is being made by means of a plough that is arranged on a device, a vertical drainage means being arranged by means of said device as well, and the horizontal drainage means are each time arranged after that until the next vertical drainage means has to be arranged. Thus both the vertical and horizontal drainage means are preferably arranged in a continuous progress of a process.
Preferably simultaneously with the arrangement of the horizontal drainage means or immediately after that, the air sealing layer is being arranged by means of the device as well, as a result of which the time of processing is further minimized.
Preferably the air sealing layer is being arranged by removing soil material from the trench walls and pressing it on the horizontal drainage means. In this way use is made of sealing material that is already present, which is particularly possible in cases of clay or loamy soil.
Alternatively the air sealing layer can be arranged by arranging a sealing foil layer or plastic material such as, for instance bentonite, on the horizontal drainage means.
Preferably the air sealing layer is arranged by means of the device.
It is further preferred that the trench is finally closed off with soil material up to approximately the original surface. Then as well no supply of extra soil material will be necessary.
In a further development of the method according to the invention the vertical drainage means are taken from a supply, and after having been arranged are separated by cutting through at a level above the trench bottom, so that the upper portion is available for contact with the preferably elongated or tubular horizontal drainage means. The cutting through preferably takes place in the device, so that a good performance is ensured.
From another aspect the invention provides an apparatus for use in forced consolidation, comprising a device provided with means for moving them in horizontal direction, over the ground surface, with means for making a trench from the ground surface down to at least just below the upper side of the earth stratum to be consolidated, with means for the stepwise supplying of a vertical drainage ribbon from a supply and driving it into the earth stratum, and means for supplying a horizontal drainage means in the trench bottom.
Preferably the device is further provided with means for cutting through the drainage ribbon at a level above the trench bottom. The means for cutting through preferably comprises a movable blade and an anvil for said blade, said blade preferably having been arranged on an arm of a lever rotatable about a horizontal center line, the other arm of said lever being connected to a vertically active hydraulic cylinder, preferably accommodated in the trench-making means. In this way a high cutting power can be achieved with low (horizontal) occupation of space, as the stroke length of the cylinder is not limited by horizontal limitations.
The trench-making means is preferably a plough, which at its rear side is provided with means for removing soil material from the trench walls and for pressing it downward, for forming an air sealing layer over the horizontal drainage means, or with means for arranging foil or bentonite, at least (initially) plastic sealing material.
Preferably, at its rear side the plough is provided with means for supplying the horizontal drainage means, particularly from a supply roll, in the trench, so that they are situated, as it were, at the free shadow side.
It is furthermore preferred that means are provided for pivoting the plough about a horizontal axis of rotation, between a trench-making active position and an upwardly tilted moving position and vice versa. On the one hand this facilitates transport, on the other hand it can promote initial penetration of the plough.